Method for making thermal insulation

ABSTRACT

A method for making a thermal insulating structure for providing thermal insulation, structural load accommodation, and noise attenuation for surfaces in aerospace systems that are exposed to high temperatures is disclosed. Structure to be protected is a high-temperature solid back sheet to which is a honeycomb core having a perforated buried septum is bonded to a substrate. A layer of cast fiber-ceramic insulation is secured to the honeycomb core, filling portions of the core outward from the septum and extending beyond the outer face of the core. To absorb structural loads, a densified top coat overlies the insulating layer. To minimize the transmission of shear-inducing loads into the insulating layer, the outer face of the honeycomb core has an irregular surface. Core cell vents extending through the walls of adjacent cells provide a controlled airspace between the back sheet and the buried septum through which a fluid coolant may be circulated.

REFERENCE TO RELATED APPLICATIONS

The present application is a divisional application based upon U.S.patent application Ser. No. 875,807, filed June 17, 1986 now U.S. Pat.No. 4,849,276, issued July 18, 1989, which was a continuationapplication based upon U.S. patent application Ser. No. 581,305, filedFeb. 17, 1984, now abandoned.

This application is also related to U.S. patent application Ser. Nos.698,496, filed Feb. 5, 1985, and 012,585, filed Jan. 9, 1987.

BACKGROUND OF THE INVENTION

The present invention relates generally to a method for making thermalinsulation for providing thermal insulation, structural loadaccommodation, and noise attenuation for aircraft, missiles, and otheraerospace systems having surfaces exposed to high temperatures.

In ducting hot exhaust gases from the engines of high-speed aircraft, itis necessary to protect the surrounding structure from excessively hightemperatures. The required protection is typically accomplished throughthe use of multilayered structures that have a refractory material onthe surface exposed to the high-temperature gases. Passageways for thecirculation of cooling fluid are often provided between the refractoryinsulation layer and the underlying substructure of the aircraft. Insuch arrangements, a refractory metal, such as columbium, may be used tofabricate the layer that encounters the exhaust gases. A substructureconsisting of one or more spaced-apart layers of superalloy underlie therefractory metal. While these arrangements are effective, they are notentirely satisfactory from the standpoint of cost and efficiency. Toprovide and circulate the coolant, it is necessary to use a portion ofthe engine output. To meet the cooling requirements of a high-poweredengine exhaust duct, a significant energy expenditure is required.Commonly, as much as 50 percent of the available output may be neededfor cooling purposes. This energy expenditure dramatically reduces theoverall efficiency of a high-shaft-power or thrust propulsion system. Afurther disadvantage of these arrangements is the added weight andexpense attributable to the materials used for the insulating layer andthe underlying substructure.

The outer surfaces of space vehicles are also subjected to hightemperatures during reentry. To protect these vehicles from the heatgenerated during reentry, various arrangements utilizing ablativematerials have been heretofore proposed, often in combination withtranspirational cooling systems. Because of their ablativecharacteristics, such systems are not well suited for insulating thesurfaces of reusable spacecraft. Accordingly, efforts have been directedtoward the development of nonablative insulating structures. In thepresent space shuttle, for example, portions of the shuttle exteriorsurface are insulated with a plurality of ceramic tiles that arearranged in a closely spaced, ordered array. To provide the requiredfit, each tile must be precision cut from a carefully formed fusedceramic blank. To form the blanks, silica fibers and other ceramiccomponents are initially mixed into a slurry and cast into blocks. Afterdrying, the blocks are sintered at high temperatures to form strongceramic bonds between the overlapping fibers. The blocks are sawnthereafter into the smaller blanks that are subsequently configured intothe final tiles by a numerically controlled mill. Once prepared eachtile is individually secured in place via a manual procedure. Thisinvolves bonding the tiles to a felt strain isolation pad with ahigh-temperature adhesive, then adhesively bonding the pad to theunderlying metallic substructure.

During takeoff and reentry, nonuniform temperature gradients existacross the surface of the space shuttle, which is insulated with theseceramic tiles. The fused ceramic structure of the tiles is poorlyresistant to shear forces, and, thus, poorly resistant to the forcesoccasioned by the differential surface temperature distribution.Accordingly, to prevent breakage of the ceramic insulation, the tilesmust be limited to small sizes, generally less than ten inches on aside. While this isolates the loads applied to the insulation, it doesso by exacting an extremely high cost in terms of parts production andassembly onto the spacecraft.

For applications other than the space shuttle, these high costs mayrender the ceramic tile approach unfeasible. Where the applicationdictates the use of an unbroken, i.e., continuous, insulation surface,the ceramic tile approach would be entirely unworkable. Since the castceramic blocks shrink substantially and in an irregular manner duringthe sintering operation, the use of such fused fibrous ceramic materialsis also not well suited for forming insulating surfaces that have asubstantial degree of curvature.

The present invention provides an arrangement that overcomes thedisadvantages of the developments described above. In particular, theinvention provides an insulating structure that not only affordsprotection against high temperature, but also accommodates loads. Thisload-bearing capability enables a reduction in the amount ofsubstructure required to support the insulation, and, thus, contributesto an overall reduction in the weight of the craft. The inventionprovides, as well, a thermal insulating structure that also functions asa noise attenuator and, thus, is particularly well suited for use in anaircraft engine exhaust duct. An important aspect of the invention isthe provision of a thermal insulating structure that is fabricated bycasting a fibrous ceramic insulation material onto a honeycomb core. Inaccordance with a particular aspect of the invention, this manufactureof the insulating structure is enabled through the use of a honeycombcore having a perforated buried septum. As a result of the supportprovided by the honeycomb core to the ceramic insulation, structureshaving continuous insulating surfaces may be fabricated in a widevariety of shapes and configurations.

SUMMARY OF THE INVENTION

In accordance with the invention, there is provided a method for makinga thermal insulator structure having a honeycomb core secured along aninner face to a substrate. The honeycomb core includes open cells thatextend between the inner and outer faces of the core. A layer ofinsulating material fills outer portions of the cells forming enclosedcavities, immediately outward from the substrate. In a preferred form,the separation of the cells into inner and outer cavities is effected bya septum that is buried within the honeycomb core in spaced relation tothe inner and outer faces thereof. In this arrangement, it is preferredthat the septum be perforated so that the insulator layer may be formedby vacuum casting a fibrous ceramic insulation material into thehoneycomb core, either before or after the core is secured to thesubstrate. Where it is desired to join the honeycomb core and substrateprior to casting the insulator layer, the substrate is selectivelyperforated and passageways are provided between the cells. The insulatorlayer is then formed by pulling the ceramic fibers from a slurry bymeans of a vacuum that is drawn on the back of the substrate and appliedvia the substrate perforations and cell passageways to the inner face ofthe perforated buried septum. Where the insulating layer is formed priorto joining the honeycomb and substrate, the vacuum is applied directlyto the face of the buried septum through the open ends of the cells thatlie along the inner face of the core.

To protect the honeycomb core and the buried septum in high-temperatureapplications, it is preferred that the insulating layer extend outwardfrom the outer face of the core. The outer face of the core preferablyhas a textured surface to minimize the transfer of loads at theinterface between the insulation layer and core. To provide structuraldurability to the insulator layer, the outer surface thereof isstrengthened or densified. In lieu of strengthening a densified top coatis optionally applied to the layer of insulating material. Where afibrous ceramic material is utilized for the insulating layer, the topcoat is preferably reinforced glass. Where noise attenuation isrequired, the top coat is provided with a controlled porosity.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention can be understood by the following portion of thespecification taken in conjunction with the accompanying drawings inwhich:

FIG. 1 is a perspective view, partially in section, showing a thermalinsulating structure in accordance with the invention; and

FIG. 2 is a side elevation view, partially in section, of the structureof FIG. 1.

DETAILED DESCRIPTION OF THE INVENTION

Referring to FIG. 1, a preferred arrangement for the thermal insulatingstructure of the invention has a substrate or back sheet 10 to which issecured a honeycomb core 12. The honeycomb core 12 has a plurality ofindividual cells 16 that are isolated from one another and arranged in ageometric pattern. A septum 14, having perforations 36, is buried, orformed internally, within the honeycomb core 12 in spaced-apart relationto the substrate 10. The septum 14 is a generally planar, continuousstructure that is configured from a plurality of individual coplanarsepta that are secured to the walls 32 of each individual cell 16 of thecore. The positioning of the septum divides the honeycomb core 12 alongits length and separates the cells 16 into inner and outer cavities 20and 22, respectively. An insulator layer 18 is secured to the honeycombcore 12, filling the outer cavities 22 of the cells and extendingoutward beyond the free edge, or outer face, 24 of the honeycomb core.The insulator layer 18 is preferably constructed of a fibrous ceramicmaterial that is vacuum cast into the honeycomb core 12 by a processmore fully described below. A densified top coat 28 having a controlledporosity covers the insulating layer 18, completing the structure andproviding the surface that is exposed in use to the high temperatures.

As will become apparent hereinafter, the invention permits themanufacture of thermal insulating structures having a wide variety ofshapes and configurations. In use, the physical configuration and otherdesign parameters of the particular application will dictate the meansand manner in which the thermal insulating structure is secured to thestructure that is to be protected from heat. For clarity ofillustration, FIGS. 1 and 2 show an exemplary arrangement that would besuitable for a large, relatively flat insulating surface. Thisinsulating structure is attached to the underlying substructure byattachment flanges, or stiffeners, 30. The number of flanges 30 and theplacement thereof are in accordance with known techniques that do notform part of this invention. It is to be noted, however, that theinsulating structure according to the invention is capable of carryingloads that would otherwise be borne by the underlying substructure. Thisaccordingly permits a reduction in the amount of substructure that isrequired and contributes to a further lightening of the craft.

The invention aims, as well, to provide a method for making a thermalinsulating structure that permits a wide latitude of design choices,both in terms of material selection and physical configuration of thecomponents. This latitude is particularly advantageous since eachapplication presents different heat characteristics. For example, inengine exhaust duct applications, the temperatures encountered are high,but relatively constant. On the other hand, for space reentry vehicleapplications, significantly higher temperatures may be encountered forshort durations during takeoff and reentry. Immediately after flight,there are different thermal problems presented by the need to dissipateheat stored in the insulating structure during reentry. To accommodatethese varied applications, the present invention permits precise controlover the thickness and density of the insulating layer 18 and placementof this layer relative to its supporting honeycomb core 12 and theunderlying back sheet 10. This is enabled in part by the wide latitudein material selection and in part through the use of a buried septum 14that facilitates manufacture of an insulator layer having uniform filldensity. The employment of the septum 14 also permits the provision of acontrolled air gap between the insulator layer 18 and back sheet 10through which a cooling fluid can be uniformly distributed if desired.Since the porosities of the insulating layer 18 and top coat 28 may alsobe controlled, the structure may be transpirationally cooled by forcingfluid outward through the structure.

With this ability to control the heat transfer through the insulatorlayer 18, it is possible to select various materials for the honeycombcore 12 and the back sheet 10. The proper selection is, once again,dependent upon the particular application. For high-temperatureapplications, suitable materials for the back sheet 10 include metals,such as steel or aluminum, and matrix composites of the types now knownand used. High-temperature polymer matrix composites are particularlydesirable since the thermal expansion characteristics of these materialsmay be matched with the expansion characteristics of other components ofthe structure in order to minimize warping.

Honeycomb core 12 also may be formed from either metallic or nonmetallicmaterials to suit the particular application. Honeycombs of this typeare well known and are typically formed from strips of nonmetallicfabric or from metallic foil or ribbons that are joined together atintervals to form geometrically shaped cells. Most often, the cells havehexagonal shapes, although other shapes are readily available or couldbe specially provided for a given use. The thickness of the cell wallsand the face-to-face height of the honeycomb are also variables that aresubject to a wide range of design choices. It is to be understood thatthe present invention contemplates usage of any cellular core withoutlimiting regard to the particular material, the size and shape of thecells, or the physical shape and dimensions of the core. Thus, the cells16 may be formed other than in hexagonal shapes and may have variousthicknesses for the cell walls 32. In a particular application, theamount of heat to be borne by the core and/or transmitted to the backsheet 10 will dictate the selection of the thickness of the insulation,and the core parameters such as material, cell cross section, cell wallthickness, and face-to-face height. To withstand or dissipatesubstantially high temperatures, it may be necessary to employ a ceramicor refractory metal core, while, in lower temperature applicationsorganic or nonrefractory metal cores may be satisfactorily used.

Since it is intended that the insulating structure accommodatestructural loads, it is preferred that measures be taken to control thesensitivity of the insulating layer to external forces. To minimize thesensitivity of the insulating layer 18 to structural flaws and tocontrol the manner in which loads are transferred to and distributedinto the insulator layer, the outer face 24 of the honeycomb core isprovided with a textured surface. The presence of a textured surfaceprovides a transition zone between the insulator layer 18 and the core.This transition zone minimizes stress concentration factors at theinterface between the honeycomb core and the insulator layer. Where thehoneycomb core 12 is constructed from glass fibers, this texturedsurface is provided by fraying the outer face 24 of the core byshredding or through the use of a chemical solvent. Mechanical orchemical means could similarly be used to notch, abrade or otherwiseprovide texture to the outer face of a metallic or ceramic core.

By casting the insulating layer 18 into the honeycomb core 12, twoimportant functional advantages are achieved. First, since structuralsupport is provided for the ceramic fibers that form the insulatinglayer 18, large, continuous insulating surfaces may be formed. This isin contrast to prior approaches in which it is necessary to fragment theinsulating surface into small pieces to prevent breakage of the fragileceramic insulator. Second, the honeycomb core 12 is easily and securelyattachable to the back sheet 10 through the use of conventionaltechniques such as adhesive bonding or brazing. This eliminatesdifficulties that have been encountered heretofore in attempting to joinan insulating layer directly to a backing substrate.

In conjunction with the septum 14, the use of honeycomb core 12 alsopermits the formation of a controlled airspace between the buried septum14 and the back sheet 10 or between the insulating layer 18 and the backsheet in instances where the buried septum is removed, as will beexplained hereinafter. To provide this airspace, the inner cavities 20of the cells are interconnected via core cell vents, or passageways, 34that pass through the cell walls 32. The airspace provided by theinterconnected cells may be used either passively or actively. Foractive cooling beneath the insulating layer 18, a suitable coolant maybe circulated through the cells by means of the core cell vents 34. Ifdesired, the insulating layer 18 and top coat 28 may be made permeableto permit passing a fluid coolant through the interconnected cells,through the perforations 36 in septum 14, and outward totranspirationally cool the structures. FIG. 1 illustrates one suitablearrangement of the core cell vents 34. It is to be understood that thecore cell vents 34 may be of a different number and shape and locatedother than as illustrated in this FIGURE. As will be explained below,the core cell vents are also used in casting the ceramic insulatinglayer 18 onto the core in the instances when the core 12 is first bondedto the backing sheet 10.

The septum 14 is formed in situ within the core 12 from a film-formingresin. A wide variety of resins is available for forming septum 14, butthose exhibiting high-temperature characteristics are preferred for mostapplications. Of these high-temperature varieties, polyimides areparticularly well suited. The process for creating the septum isinitiated by partially embedding the core with a suitable material, suchas wax, which has a lower melting point than the selected film-formingresin. The depth of the embedded core is selected in accordance with thedesired location of the septum, which, in turn, is dictated inaccordance with the desired positioning of the insulating layer relativeto the back sheet 10 in the final structure. Depending upon theapplication, the septum may be located anywhere between the inner andouter extreme faces 24 and 26, respectively of the honeycomb core 12. Toassure a clean bond between the core and back sheet 10, it is preferredthat the septum be located at least some distance away from the inneredge or face 26 of the honeycomb core.

To form the septum, the embedded wax or other material is covered withan uncured film of resin. After the resin has cured, the wax or othermaterial is removed, leaving the cured resin as a septum that is buriedor suspended internally within the honeycomb core 12, i.e., positioned"through" the cells in spaced relation to the outer and inner faces, 24and 26, respectively. The septum is then laser drilled to provide theperforations 36 required for vacuum casting the insulator layer 18. Theprocess just described is similar to the process for makingsound-insulating panels described in commonly assigned U.S. Pat. No.4,257,998 to Diepenbrock, Jr. et al. According to this latter process, aseptum material is spread on top of a destructible mold, the top of themold having upstanding studs that penetrate through the septum material.A cellular core is pressed through the septum material and into thematerial of the destructible mold until the septum is internallypositioned where desired within the core. After the septum is hardened,the destructible mold is removed from the core, leaving the septum inplace. Such a process is a suitable alternative for forming the septum14 of the instant invention. Further details thereof may be obtained byreference to this U.S. Pat. No. 4,257,998, the disclosure of which ishereby incorporated by reference. Further details of the structureproduced thereby may be obtained by reference to commonly assigned U.S.Pat. No. 4,265,955 to Harp et al., the disclosure of which is herebyincorporated by reference.

As mentioned above, the presence of the perforations 36 also permitsventing a coolant outward through the insulating layer 18 fortranspirational cooling purposes. The number and size of theperforations 36 are selected in accordance with the needs of theapplication. For example, where different sections of the sameinsulating structure are subjected to differential pressures, as in along exhaust duct, it may be desirable to configure the perforations 36to function as auxiliary routes for pressure relief for the open innercavities 20 of the cells. On the other hand, where noise attenuation isa prime concern, smaller and fewer perforations would be desirable.

As briefly noted above, the insulator layer 18 is formed by casting aceramic insulating material into the outer cavities 22 of the honeycombcore. While a variety of casting processes may be employed, it ispreferable to use a vacuum casting process in which the insulator layeris formed by pulling a ceramic fiber-containing slurry through the openouter face of the core by means of a vacuum drawn on the underside ofthe buried septum 14.

The insulator layer 18 may be formed onto the honeycomb core 12 eitherbefore or after bonding the core to the back sheet 10. When thehoneycomb core and back sheet are joined prior to formation of theinsulating layer, provision must be made for applying a vacuum throughthe back sheet 10 into the inner cavities 20 of the cells. For thispurpose, back sheet 10 is provided at selective intervals with vacuumports 38. Since the inner cavities 20 of the cells are in fluidcommunication via the core cell vents 34, vacuum may be applieduniformly to the underside of the septum 14. For purposes of clarity,the attachments needed for vacuum processing have not been shown in theFIGURES. The means and manner of providing such attachments will, ofcourse, be readily apparent to those familiar with the art of ceramicvacuum casting.

Where the insulating layer 18 is formed onto the core prior to bondingthe core to the backing sheet, the vacuum is applied directly throughthe open ends of the cells. In this instance, the core is fitted withina suitable vacuum fitting.

For most applications, it is preferred that the outer surface of theinsulator layer 18 be strengthened somewhat in order to absorbstructural loads. This may be accomplished by densifying the outersurface of the insulator layer 18 as, for example, through heattreatment of the ceramic insulating material or through selection andcontrol of the process variables during formation of the final thicknessof the layer. Alternately, and as illustrated in the FIGURES, a separatetop coat 28 may be formed over the insulator layer 18 through theaddition of secondary materials. It will be appreciated that a widerange of fibrous, glassy, and particulate materials and mixtures andsolutions thereof are suitable for this purpose.

A particularly desirable material is a ceramic material that is easilyfused to the underlying insulator layer 18. For added durability, thetop coat may optionally include some reinforcement. A mesh-reinforcedglass satisfies both of these criteria and is, thus, preferred.

It is particularly desirable to control the porosity of the densifiedtop coat to permit taking advantage of the noise-attenuating nature ofthe porous ceramic insulating layer 18. By providing sufficient porosityto the top coat 28, the thermal insulating structure may also usefullyprovide noise abatement. This is particularly useful for applicationssuch as engine exhaust ducts for commercial aircraft auxiliary powerunits. The means and manner of controlling the porosity of the top coatwill be apparent to those skilled in the art.

While the septum 14 is required during the formation of the insulatinglayer 18, there may be instances in which it is desirable to remove theseptum. For example, where high temperatures are to be encountered bythe structure, undesired outgassing of the organic septum may occur. Toprevent this, the septum may be removed prior to bonding the honeycombinsulator assembly to the back sheet 10. The means used to effectremoval will, of course, depend upon the material used for the septum.By way of example, a polyimide septum might be removed by chemical orthermal means.

The present invention has been described in relation to its preferredembodiments. One of ordinary skill, after reading the foregoingspecification, will be able to effect various changes and substitutionsof equivalents without departing from the broad concepts disclosedherein. It is therefore intended that the protection afforded by LettersPatent granted hereon be limited only by the definition contained in theappended claims and equivalents thereof.

The embodiments of the invention in which an exclusive property orprivilege is claimed are defined as follows:
 1. A method for making aceramic insulation, comprising the steps of:(a) forming a honeycomb corewith a textured surface on at least one face, wherein the texturedsurface is formed by fraying the face of the core; (b) forming a ceramicinsulation layer in an upper portion of each cell of the honeycomb coreby vacuum casting a slurry of ceramic fibers into the core; (c) forminga transition zone in the insulation by continuing to case the slurryover the textured surface; (d) continuing to cast an integral layer ofceramic fiber insulation to extend in a sheet over the transition zone;and (e) adhering a reinforced glass layer to the sheet.
 2. The method ofclaim 1 wherein fraying is achieved by shredding the core.
 3. The methodof claim 1 wherein fraying is achieved by exposing the core to achemical solvent.
 4. The method of claim 1 wherein step (b) includesdrawing a vacuum through perforations in a septum positioned in eachcell to draw the slurry into the core and wherein step (d) includescontinuing to draw the vacuum through the septum and layer in the coreto cast the sheet.
 5. The method of claim 4 further comprising the stepof removing the septum from the core.
 6. The method of claim 1 furthercomprising (1) adhering the core to a substrate including at least oneport in fluid communication with at least one cell and each cellincluding a perforated septum and at least one core cell vent for fluidcommunication between the cells; and (2) drawing the vacuum through theport, the core cell vents, and the septum to draw the slurry into thecore adjacent the septum.
 7. The method of claim 6 further comprisingremoving the septum after casting the sheet.
 8. The product of theprocess of claim
 7. 9. The product of claim 8 wherein the core isconstructed from glass fibers.
 10. The product of claim 8 wherein thecore is metal.
 11. A method for making a ceramic insulation, comprisingthe steps of:(a) forming a honeycomb core with a textured surface on atleast one face, wherein the textured surface is notched; (b) forming aceramic insulation layer in an upper portion of each cell of thehoneycomb core by vacuum casting a slurry of ceramic fibers into thecore; (c) forming a transition zone in the insulation by continuing tocast the slurry over the textured surface; (d) continuing to cast anintegral layer of ceramic fiber insulation to extend in a sheet over thetransition zone; and (e) adhering a reinforced glass layer to the sheet.12. The method of claim 11 wherein the textured surface is abraded. 13.The method of claim 12 further comprising (1) adhering the core to asubstrate prior to step (b), the substrate including at least one portin fluid communication with at least one cell and each cell including atleast one core cell vent for fluid communication between the cells; and(2) drawing the vacuum through the port and core cell vents to draw theslurry into the core.
 14. The product of the process of claim
 13. 15.The method of claim 11 further comprising (1) adhering the core to asubstrate prior to step (b), the substrate including at least one portin fluid communication with at least one cell and each cell including atleast one core cell vent for fluid communication between the cells; and(2) drawing the vacuum through the port and core cell vents to draw theslurry into the core.
 16. The product of the process of claim
 15. 17. Amethod for making a ceramic insulation, comprising the steps of:(a)forming a honeycomb core with a textured surface on at least one facewherein the textured surface is formed by a method selected from thegroup consisting of fraying, shredding, notching, or abrading; (b)forming a ceramic insulation layer in an upper portion of each cell ofthe honeycomb core by vacuum casting a slurry of ceramic fibers into thecore; (c) forming a transition zone in the insulation by continuing tocast the slurry over the textured surface; and (d) forming a top fibrousinsulation sheet by continuing to cast the slurry over the transitionzone.
 18. The method of claim 17 further comprising the step ofstrengthening an outer portion of the sheet.
 19. The method of claim 18wherein strengthening includes adhering glass to the sheet.